1. Field of the Invention
The present invention relates to an image forming apparatus comprising means for cleaning a surface of a member with a movable cleaning member maintained in contact with said surface, and more particularly to such apparatus in which the movement of said cleaning member is variably controlled in response to the status of image formation.
2. Description of the Prior Art
In the following explanation an electrophotographic copier will be taken as an example of the image forming apparatus.
In such electrophotographic copier there is already known the use of a cleaning member such as a cleaning roller for eliminating deposited substances such as toner particles remaining on a member, and particularly widely used is a web cleaning device utilizing a cleaning web composed for example of non-woven cloth. FIG. 1 is a side view of such a conventional web cleaning device applied in the cleaning of a fixing device.
In FIG. 1 there is provided a pressure roller 3 of an elastic member rotatably supported to maintain a cleaning web 2 composed for example of non-woven cloth in contact with a rotatably supported fixing roller 1, and said cleaning web 2 is supplied from an unwind shaft 5 and is taken up on a driven winding shaft 4. An arrow 6 indicates the rotating direction of the fixing roller 1, and an arrow 7 indicates the advancing direction of the cleaning web 2. A copy sheet P bearing an unfixed image on the upper face thereof advances on a guide member 8 and is introduced between the fixing roller 1 provided therein with a heat source and a pressure roller 9 rotatably supported and maintained in contact with said fixing roller 1, whereby the unfixed image is fixed to the copy sheet by the heat and pressure. Thereafter the copy sheet P is ejected to the outside through a pair of ejecting rollers 10 rotated in mutually opposite directions. In the course of the above-described procedure, a part of the toner 11 constituting the image on the copy sheet P becomes adhered to the fixing roller 1, and such adhesion is called the offset phenomenon. The toner 11 thus deposited on the fixing roller 1 moves as indicated by the arrow 6 and is wiped away by the cleaning web 2 moving at a speed much lower than that of the fixing roller 1.
In such web cleaning device, the advancing amount of such web per unit time is selected in such a manner as to perform complete cleaning even in a continuous image forming operation. Consequently the web can be advanced by the necessary amount for each image forming operation. However, in ordinary copiers, the heat capacity of the heater of the fixing device has a limitation so that the temperature of the fixing roller becomes lower than the determined value in a continuous imaging operation, thus increasing the toner deposition on the fixing roller. Such phenomenon becomes more pronounced as the ambient temperature of the copier becomes lower.
(i) In this manner, the amount of toner deposited on the fixing roller is dependent on the temperature of the fixing roller, and, if the advancing amount of the cleaning web is fixed as in the conventional device independent from the temperature of the fixing roller, complete cleaning of the fixing roller may not be possible when the temperature thereof becomes significantly lower than the predetermined value in a continuous imaging operation. On the other hand, if the advancing amount of the cleaning web is made larger in order to avoid such drawback, the web is wasted as long as the fixing roller is maintained at the predetermined temperature, thus requiring frequent replenishments.
(ii) Also in case the image density is low, the cleaning web is wastedly advanced although there is sufficient cleaning ability. On the other hand, in case the image density is high, complete cleaning may not be possible because the cleaning ability of the web is limited.
(iii) In the conventional web cleaning device, the advancement of the cleaning web is synchronized with the rotation of the main motor and/or the fixing roller. As already known, the main motor has to perform pre-and post-rotation steps in order to attain uniform surface potential on the photosensitive drum, and the fixing roller has to perform a pre-rotation step until it reaches a predetermined temperature for obtaining uniform temperature distribution so that the main motor and/or fixing roller must be operated beyond the necessary time period for copying operation.
Consequently, during these steps not included in the actual copying operation, the web has to be wastefully advanced, thus requiring frequent replenishment.
(iv) In the web cleaning device for a copier, the advancing amount of the web per unit time can generally be fixed at a predetermined value. However, in a copier in which the peripheral speed of the photosensitive drum changes for equal-size copying and reduced-size copying, for example from a high speed of 270 mm/sec at the equal size copying to a low speed of 180 mm/sec at the reduction copying, such web cleaning device is associated with the following drawbacks. If the advancing amount of the cleaning web per unit time is so selected to completely remove the toner deposited on the fixing roller at the low peripheral speed of the photosensitive drum, the web cleaning device may not be able to achieve complete cleaning at the copying operation with the high peripheral speed of the photosensitive drum. On the other hand, if said advancing amount is so selected to completely remove the deposited toner at the high peripheral speed of the photosensitive drum, the cleaning device will result in a waste of the cleaning web in the copying operation with the low peripheral speed of the photosensitive drum, requiring frequent replacement of the cleaning web.